Mr. ElBaradei’s emergence as an opposition figure is especially surprising given that when he stepped down in November 2009 after a dozen years heading the U.N’.s International Atomic Energy Agency, he expressed no interest in becoming involved in Egyptian politics. “When people were first approaching him saying, ‘Will you run for president of Egypt in 2011?’ he was very dismissive of it,” says Laban Coblentz, Mr. ElBaradei’s longtime speechwriter, who recently helped him write a memoir slated to be published in April.

But ex-colleagues say Mr. ElBaradei, whose international profile soared after he shared the Nobel Peace Prize with the IAEA in 2005, experienced a change of heart after teaching himself how to use social-networking sites on the Internet to monitor Egyptian events from afar.

Among other things, he discovered Facebook “fan” pages with thousands of followers urging him to run for president this year. His own Facebook page, which is frequently updated, currently has more than 314,000 admirers.

“It was really this last 14 months, where someone I knew as not being particularly computer savvy, taught himself to use Facebook and Twitter and YouTube and started to do in virtual space which was forbidden to do by the Mubarak regime, the freedom of assembly by large groups,” says Mr. Coblentz.